The invention relates to using scanning interferometry to measure surface topography and/or other characteristics of objects having complex surface structures, such as thin film(s), discrete structures of dissimilar materials, or discrete structures that are underresolved by the optical resolution of an interference microscope. Such measurements are relevant to the characterization of flat panel display components, semiconductor wafer metrology, and in-situ thin film and dissimilar materials analysis.
Interferometric techniques are commonly used to measure the profile of a surface of an object. To do so, an interferometer combines a measurement wavefront reflected from the surface of interest with a reference wavefront reflected from a reference surface to produce an interferogram. Fringes in the interferogram are indicative of spatial variations between the surface of interest and the reference surface.
A scanning interferometer scans the optical path length difference (OPD) between the reference and measurement legs of the interferometer over a range comparable to, or larger than, the coherence length of the interfering wavefronts, to produce a scanning interferometry signal for each camera pixel used to measure the interferogram. A limited (or “low”) coherence length can be produced, for example, by using a broadband light source (e.g., a white light source), which is referred to as scanning white light interferometry (SWLI). A typical scanning white light interferometry (SWLI) signal is a few fringes localized near the zero optical path difference (OPD) position. The signal is typically characterized by a sinusoidal carrier modulation (the “fringes”) with bell-shaped fringe-contrast envelope. The conventional idea underlying SWLI metrology is to make use of the localization of the fringes to measure surface profiles. Low-coherence interferometry signals can also be produced with narrow band light that illuminates an object over a wide range of angles.
Techniques for processing low-coherence interferometry signals include two principle trends. The first approach is to locate the peak or center of the envelope, assuming that this position corresponds to the zero optical path difference (OPD) of a two-beam interferometer for which one beam reflects from the object surface. The second approach is to transform the signal into the frequency domain and calculate the rate of change of phase with wavelength, assuming that an essentially linear slope is directly proportional to object position. This latter approach is referred to as Frequency Domain Analysis (FDA). In the presence of thin film structures, the analysis can be more complicated.
U.S. patent applications published as US-2005-0078318-A1 entitled “METHODS AND SYSTEMS FOR INTERFEROMETRIC ANALYSIS OF SURFACES AND RELATED APPLICATIONS” and US-2005-0078319-A1 entitled “SURFACE PROFILING USING AN INTERFERENCE PATTERN MATCHING TEMPLATE, both by Peter J. de Groot, disclose additional techniques for analyzing low-coherence interferometry signals from a thin film sample. One of the disclosed techniques identifies the portion of a scanning white light interferometry (SWLI) signal corresponding to the top-surface profile of a thin film structure. For a thin enough film, the individual signals corresponding to the upper and lower interfaces of the film are inseparable, in the sense that the fringe contrast has only one peak; nonetheless, we can argue on physical grounds that the first few fringes on the right most closely relate to the top-surface profile. This technique identifies the trumpet-shaped leading edge of the signal, and ascribes this to the top surface profile. A further technique disclosed in these published applications describes one way of locating the leading edge or other segment of a signal by using a pattern matching technique, one example of which is referred to as correlation template analysis (CTA). Both of said published applications are commonly owned with the present applications and are incorporated herein by reference.